[etherswitch] Support Micrel KSZ8995MA switch chip

This is Micrel KSZ8995MA driver code. KSZ8995MA uses SPI bus to control.
This code is written & tested on @SRCHACK's ksz8995ma board and FON2100
with gpiospi.
etherswitchcfg support commands: addtag, ingress, striptag, dropuntagged.

Submitted by: Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by: mizhka, adrian
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D8790